Occurrence, genetic diversity, and control of Salmonella enterica in native Brazilian farmed fish

Abstract

Bacteria in the genus Salmonella are some of the most important foodborne pathogens worldwide. Despite their broad distribution, there are few records on these microorganisms in poikilothermic animals. Recently, Salmonella spp. were detected in the carcasses of native fish in Brazilian slaughterhouses, presenting the risk of foodborne-related salmonellosis. The aims of this study were to evaluate the occurrence, main serotypes, and genetic diversity of S. enterica in native farmed fish, as well as to determine appropriate control strategies. Five hundred and sixty-nine samples of rations, pond water, gill swabs, and feces of native fish were collected from commercial farms for bacteriological studies. The Salmonella isolates were identified using PCR and serotyped using a conventional method. Genetic diversity was evaluated by multi-locus sequence typing (MLST). Three orally administered products were tested to control Salmonella in experimentally infected fish: a probiotic; organic acids; and humic acid. Thirty-nine S. enterica isolates were obtained from water and fish samples. The isolates were classified as serotypes Brandenburg, Hadar, Heidelberg, Panama, and Saintpaul. One isolate could not be serotyped. Using MLST, the isolates were shown to represent 26 sequence types (STs), with 21 of these STs described here for the first time and five STs that were previously characterized. In the treatment trial, the groups of fish given feed with a probiotic and organic acids showed a 50% reduction in the number of fish positive for S. Hadar until the 20th day post-challenge. However, none of these products was able to clear S. Hadar from the guts of tambaqui. The strains of Salmonella enterica from fish showed high serotype and genetic diversity, which suggests multiple sources of infection and the absence of host-specific strains. The probiotic and organic acids might be used strategically prior to harvest to reduce Salmonella infection in fish herds, thereby decreasing the potential for carcass contamination.